1. Field of the Invention
This invention relates to primary medical devices for the reception of radio frequency electromagnetic radiation, particularly medical devices used to obtain a magnetic resonance image of a region in front of the device and within a natural organism or patient (such as within a human) or elsewhere, and secondary medical devices such as catheters and secondary devices for delivery of therapeutic agents and monitoring of metabolic activity. The use of magnetic resonance primary medical devices to provide enhanced imaging within the region of interest in conjunction with the deployment of secondary medical devices offers a particularly effective means of delivering targeted therapy.
2. Background of the Invention
In the practice of the present invention, the term MR microcoil is used to denote a magnetic resonance device used for imaging internally from a patient. This term is in contrast to MR coils that are conventionally used externally to the body for MR imaging purposes. The MR microcoil may be mounted at the tip of a catheter or other insertion device used to probe the interior of a body. The combination of the microcoil mounted on another device provides quick and direct access to the region where imaging is required. Medical procedures such as image-guided and minimal access surgery, performed within small regions of a patient""s anatomy, demand the ability to visualize the internal terrain and/or the procedure being performed by the surgeon. When the secondary medical device is also intentionally altering the molecular content in the neighborhood of the anatomical region being treated or infused with therapeutics, it is also important to be able to determine the direction and amount of change in the molecular content. While alternative methods, including x-ray imaging and fiber optic viewing offer possible alternative means of performing the visualization of terrain and the location of physical secondary devices, magnetic resonance imaging methods are a particularly convenient means of doing this, especially given the highly localized nature of the procedures being performed. In addition, as described in U.S. patent applications Ser. Nos. 08/857,043 and 08/856,894 filed on May 15, 1997, the use of improved Magnetic Resonance Imaging (MRI) techniques and devices enables a real-time visualization of compositional changes in the molecular composition of small regions within patients. The compositional changes may be caused by delivery of drugs or active chemicals, or by the stimulation of local chemical production by tissues or organs in the patient. MRI can actually enable visualization of minute concentration changes within the body, particularly intracranial regions of the patient.
Limitations exist with other imaging modalities, such as x-rays or fiber optics. For example, extended x-ray exposures are harmful to the patient, and fiber optic equipment is not well suited either to viewing within small confines or to volume visualization. Both of these limitations in the alternative technologies are circumvented by magnetic resonance imaging.
U.S. Pat. No. 5,271,400 describes a tracking system for the position and orientation of an invasive device within a patient. The device includes a receiver coil and an MR active sample. The receiver picks up magnetic resonance signals generated by the sample. The frequencies are proportional to the location of the coil along the applied field gradients, since the signals are received in the presence of these magnetic field gradients. The system is designed to enable location of the invasive device and enhanced imaging of a region around the invasive device is not a functionality intended for this device.
In xe2x80x98MR imaging of blood vessels with an intravascular coilxe2x80x99, J. Mag. Res. Imag., 1992, Vol.2, pages 421-429, A. J. Martin, D. B. Plewes and R. M. Henkelman describe an opposed solenoid design for an intravascular MR microcoil. This paper describes microcoils made of a pair of helical windings arranged in opposed fashion at the tip of a catheter, shown to be suitable for magnetic resonance imaging purposes. The term xe2x80x9copposed coilxe2x80x9d means a coil in which the relative winding of two coil segments is opposite in sense, and the current flow in each opposed coil winds in opposite directions about the coil axis (relative to moving towards or away from the core or axis of the coil). That is, viewing the coils looking down an axis of the core around which the coils are disposed, one will be wrapped clockwise and the other will be wrapped counterclockwise, with a common lead between the two segments. The field-of-view of this coil is roughly cylindrical about the opposed solenoidal windings. The coil is essentially radio frequency insensitive beyond the longitudinal extent of the windings since the magnetic field in this design is squeezed out of the gap between the windings and is only significantly large in a cylindrical region that does not extend too far beyond this gap.
E. Atalar et al. describe a catheter receiver coil in xe2x80x98High resolution MRI and MRS by using a catheter receiver coilxe2x80x99, Mag. Res. Med., 1996, Vol. 36, pages 596-605. This design uses essentially a long wire looped back on itself inside a catheter so that the conductor functions in effect as a transmission line. The sensitivity of this receiver design is affected by the conductor length. In addition, the region of sensitivity surrounds the coil, and does not extend forward beyond the coil.
U.S. Pat. No. 4,572,198 describes a catheter for use with magnetic resonance imaging systems. The catheter includes a wound coil for exciting a weak magnetic field at the catheter tip. This structure provides a local distortion of the MR image, yielding an image cursor on the magnetic resonance imaging display. This design is not intended for high-gain applications.
In U.S. Pat. No. 5,964,704, Truwit and Liu discuss an opposed solenoid design for an MR microcoil with helical windings whose pitch varies along the length of the winding with the aim of achieving homogeneity or control of the field generated by the coils. The field of view is not addressed in this patent document, although the central region of view is enhanced and may be controlled by application of the principles described in that patent.
A copending, commonly assigned U.S. patent application Ser. No. 09/532,145 filed the same date as this application and entitled xe2x80x9cA Device for High-Gain and Uniformly Localized magnetic Resonance Imagingxe2x80x9d by R. Viswanathan and R. Raghavan describes a horn-shaped coil designed to maximize receptive field homogeneity within the field of view (which is a roughly cylindrical region surrounding the coil), which field of view does not extend forward beyond the coil.
A microcoil device for producing a very high-gain signal in a wide field of view in a cylindrical region surrounding the coil, but not forward to it, was described by R. Viswanathan in a second U.S. patent application Ser. No. 09/532,667.
Although recent technology has clearly advanced the image enhancing capability of secondary medical devices in regions perpendicular to the axis of the core of the windings on the secondary medical device, there is always further need in the art to expand the field and regions where images can be enhanced.
While microcoils for internal imaging have been described before, the device of the present invention advances the state of the art by design features that maximize the field of view in a direction forward to and beyond the spatial extent of the coil itself (e.g., parallel to the axis or the core of the device), as well as improving the signal gain within this field of view. The signal power falls off with distance in the forward direction (forward being defined as a direction outwardly directed from the device along the core axis of the microcoils). For volume imaging purposes, this fall-off can be adjusted for by dividing the reconstructed image intensity at a given voxel location by the gain corresponding to that voxel (which is proportional to the magnetic field B at that location), and repeating this for all voxels.
An aspect of the invention comprises a device to be inserted into a patient comprising a solid body having at least one microcoil, or at least one pair of radially opposed microcoils physically associated with the solid body, each microcoil having an outside microcoil diameter of 6 mm or less, all individual windings of said each microcoil together defining a geometric plane of each microcoil, and the plane of each microcoil being parallel to the plane of another microcoil in each pair of radially opposed microcoils.